iphone - UITableView自定义单元格非常慢

Question

我有一个 UITableView，并且创建了一个自定义单元格来显示我的表格。我显示了 6 个 UILable，虽然我只有 20 条记录要显示，但滚动时速度非常慢。

这就是我的 - tableView: cellForRowAtIndexPath: 的样子:

- (UITableViewCell *)tableView:(UITableView *)tableView
         cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    static NSString *CustomCellIdentifier = @"CustomCellIdentifier";

    HistoryCell *cell = (HistoryCell *)[tableView dequeueReusableCellWithIdentifier: CustomCellIdentifier];

    if (cell == nil) { 
        NSArray *nib = [[NSBundle mainBundle] loadNibNamed:@"HistoryCell" owner:nil options:nil];

        for (id oneObject in nib)
            if ([oneObject isKindOfClass:[UITableViewCell class]])
                cell = (HistoryCell *) oneObject;
    }

    NSArray *object;
    object = [cours objectForKey: [NSString stringWithFormat:@"%d", indexPath.section]];
    History *rowData = [object objectAtIndex:indexPath.row];

    if (rowData.month == 99) {
        cell.hour.frame = CGRectMake(10, 0, 135, 35);
        cell.data.hidden = YES;
        cell.hour.textColor = [UIColor blackColor];
        cell.hour.font = [UIFont fontWithName:@"Verdana" size:17];
    } else {
        cell.data.hidden = NO;
        cell.hour.frame = CGRectMake(10, 16, 135, 19);
        cell.hour.textColor = [UIColor grayColor];
        cell.hour.font = [UIFont fontWithName:@"Verdana" size:12];
    }

    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:@"d (EEEE)"];
    [formatter setLocale:self.selectedLanguageLocale];
    NSString *stringFromDate = [formatter stringFromDate:rowData.data];
    [formatter release];

    cell.data.text = stringFromDate;
    cell.hour.text = rowData.ora;

    float Var1  = [rowData.Var2 floatValue];
    float Var2  = [rowData.Var2 floatValue];

    cell.R1.text = [self floatToStringFormat: [rowData.R1 floatValue]];
    cell.R2.text = [self floatToStringFormat: [rowData.R2 floatValue]];

    if (Var1 <= 0) {
        cell.Var1.textColor = [UIColor greenColor];
    } else {
        cell.Var1.textColor = [UIColor redColor];
    }
    if (Var2 <= 0) {
        cell.Var2.textColor = [UIColor greenColor];
    } else {
        cell.Var2.textColor = [UIColor redColor];
    }
    cell.Var1.text = [self floatToStringFormat:Var1];
    cell.Var2.text = [self floatToStringFormat:Var2];

    cell.selectionStyle = UITableViewCellSelectionStyleGray;
    cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;

    return cell;
}

滚动运行如此缓慢的原因是因为我在这里所做的所有事情(NSDateFormatter、CGMakeRect、floatToStringFormat...)或者重用单元格有问题吗？

floatToStringFormat 是将数字格式化为 4 位小数的函数:

- (NSString *)floatToStringFormat:(float)number{
    NSNumberFormatter *myFloat = [[NSNumberFormatter alloc] init]; 
    [myFloat setFormatterBehavior:NSNumberFormatterBehavior10_4]; 
    [myFloat setNumberStyle:NSNumberFormatterDecimalStyle];
    [myFloat setRoundingMode:NSNumberFormatterRoundHalfUp];
    [myFloat setMinimumFractionDigits:4];
    [myFloat setMaximumFractionDigits:4];
    NSString *res = [myFloat stringFromNumber:[NSNumber numberWithFloat:number]];
    [myFloat release];
    return res;
}

Answer 1

创建和设置格式化程序对象确实是一项昂贵的操作，因此我会从重用格式化程序对象开始，因为它们在每个函数调用中都是相同的。因此，要么在数据源类中将它们设为静态变量或即时变量，然后按以下方式创建:

//static variable case
NSDateFormatter *formatter = nil;
if (!formatter){
   formatter = [[NSDateFormatter alloc] init];
   [formatter setDateFormat:@"d (EEEE)"];
   [formatter setLocale:self.selectedLanguageLocale];
}
NSString *stringFromDate = [formatter stringFromDate:rowData.data];
...